How do I add a shortcut to add a modifier to an object?
As with a lot of things in blender, you do this by right-clicking on a menu item and choosing Add Shortcut. In this instance, open the Add Modifier menu (the menu found on objects where you choose which modifier to add), right-click on the modifier you want to add a shortcut to, choose Add Shortcut and then press the keys you want to assign to the modifier:



Alternatively, if you want to do this via the User Preferences you can go to the Input tab of the preferences, open the 3D View> Object Mode category and click Add New.

In the operator id box you put object.modifier_add as this is the command that is triggered when adding a modifier. You can then select the type of modifier you want to add from the Type dropdown box and then of course specify the keys you want for this shortcut: